e81b75f7b2
SYNC in __switch_to() is a historic relic and not needed at all. - In UP context it is obviously useless, why would we want to stall the core for all updates to stack memory of t0 to complete before loading kernel mode callee registers from t1 stack's memory. - In SMP, there could be potential race in which outgoing task could be concurrently picked for running on a different core, thus writes to stack here need to be visible before the reads from stack on other core. Peter confirmed that generic schedular already has needed barriers (by way of rq lock) so there is no need for additional arch barrier. This came up when Noam was trying to replace this SYNC with EZChip specific hardware thread scheduling instruction for their platform support. Link: http://lkml.kernel.org/r/20151102092654.GM17308@twins.programming.kicks-ass.net Cc: Peter Zijlstra <peterz@infradead.org> Cc: linux-kernel@vger.kernel.org Cc: Noam Camus <noamc@ezchip.com> Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
115 lines
3.3 KiB
C
115 lines
3.3 KiB
C
/*
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* Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*
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* Vineetg: Aug 2009
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* -"C" version of lowest level context switch asm macro called by schedular
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* gcc doesn't generate the dward CFI info for hand written asm, hence can't
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* backtrace out of it (e.g. tasks sleeping in kernel).
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* So we cheat a bit by writing almost similar code in inline-asm.
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* -This is a hacky way of doing things, but there is no other simple way.
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* I don't want/intend to extend unwinding code to understand raw asm
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*/
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#include <asm/asm-offsets.h>
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#include <linux/sched.h>
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#define KSP_WORD_OFF ((TASK_THREAD + THREAD_KSP) / 4)
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struct task_struct *__sched
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__switch_to(struct task_struct *prev_task, struct task_struct *next_task)
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{
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unsigned int tmp;
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unsigned int prev = (unsigned int)prev_task;
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unsigned int next = (unsigned int)next_task;
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__asm__ __volatile__(
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/* FP/BLINK save generated by gcc (standard function prologue */
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"st.a r13, [sp, -4] \n\t"
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"st.a r14, [sp, -4] \n\t"
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"st.a r15, [sp, -4] \n\t"
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"st.a r16, [sp, -4] \n\t"
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"st.a r17, [sp, -4] \n\t"
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"st.a r18, [sp, -4] \n\t"
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"st.a r19, [sp, -4] \n\t"
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"st.a r20, [sp, -4] \n\t"
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"st.a r21, [sp, -4] \n\t"
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"st.a r22, [sp, -4] \n\t"
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"st.a r23, [sp, -4] \n\t"
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"st.a r24, [sp, -4] \n\t"
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#ifndef CONFIG_ARC_CURR_IN_REG
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"st.a r25, [sp, -4] \n\t"
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#else
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"sub sp, sp, 4 \n\t" /* usual r25 placeholder */
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#endif
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/* set ksp of outgoing task in tsk->thread.ksp */
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#if KSP_WORD_OFF <= 255
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"st.as sp, [%3, %1] \n\t"
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#else
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/*
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* Workaround for NR_CPUS=4k
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* %1 is bigger than 255 (S9 offset for st.as)
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*/
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"add2 r24, %3, %1 \n\t"
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"st sp, [r24] \n\t"
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#endif
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/*
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* setup _current_task with incoming tsk.
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* optionally, set r25 to that as well
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* For SMP extra work to get to &_current_task[cpu]
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* (open coded SET_CURR_TASK_ON_CPU)
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*/
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#ifndef CONFIG_SMP
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"st %2, [@_current_task] \n\t"
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#else
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"lr r24, [identity] \n\t"
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"lsr r24, r24, 8 \n\t"
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"bmsk r24, r24, 7 \n\t"
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"add2 r24, @_current_task, r24 \n\t"
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"st %2, [r24] \n\t"
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#endif
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#ifdef CONFIG_ARC_CURR_IN_REG
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"mov r25, %2 \n\t"
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#endif
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/* get ksp of incoming task from tsk->thread.ksp */
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"ld.as sp, [%2, %1] \n\t"
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/* start loading it's CALLEE reg file */
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#ifndef CONFIG_ARC_CURR_IN_REG
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"ld.ab r25, [sp, 4] \n\t"
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#else
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"add sp, sp, 4 \n\t"
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#endif
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"ld.ab r24, [sp, 4] \n\t"
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"ld.ab r23, [sp, 4] \n\t"
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"ld.ab r22, [sp, 4] \n\t"
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"ld.ab r21, [sp, 4] \n\t"
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"ld.ab r20, [sp, 4] \n\t"
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"ld.ab r19, [sp, 4] \n\t"
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"ld.ab r18, [sp, 4] \n\t"
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"ld.ab r17, [sp, 4] \n\t"
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"ld.ab r16, [sp, 4] \n\t"
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"ld.ab r15, [sp, 4] \n\t"
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"ld.ab r14, [sp, 4] \n\t"
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"ld.ab r13, [sp, 4] \n\t"
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/* last (ret value) = prev : although for ARC it mov r0, r0 */
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"mov %0, %3 \n\t"
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/* FP/BLINK restore generated by gcc (standard func epilogue */
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: "=r"(tmp)
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: "n"(KSP_WORD_OFF), "r"(next), "r"(prev)
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: "blink"
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);
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return (struct task_struct *)tmp;
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}
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